1. Field of the Invention
The present invention relates to a polyester film for sublimation type thermal transfer printing. In particular, the present invention relates to a polyester film for sublimation type thermal transfer printing, which has a specific coating so that it is excellent in adhesion to a sublimation type ink layer, causes less shifting of the ink layer onto an image-receiving paper sheet, and forms a good transferred image having less dyeing of said coating layer with a sublimation type dye.
2. Description of the Related Art
A thermal transfer printing system has been widely spread since it generates clear images or characters, and also it uses a simple equipment and generates less or no noise. In particular, a sublimation ink type thermal transfer printing system is increasingly used mainly for image copying such as a video printer, since it creates better gradation than a meltable ink type one.
A binder to be contained in a sublimation type ink layer has an intimate relationship with performances of a thermal transfer printing material. If a binder which makes sublimation of the ink easy is selected, adhesion between the sublimation type ink layer and a polyester film as a substrate is deteriorated to cause a trouble that an area of the ink layer is often shifted to an image-receiving paper sheet in a thermal transfer printing step.
To improve the adhesion between the ink layer and the polyester film, it has been proposed to form a thin film at an interface therebetween by, for example, coating. However, the formation of such thin film is not easy in an industrial scale production. Particularly in these years, from economical and environmental view points, importance of recyclability is emphasized. But, the formation of a coating layer suitable for this purpose is difficult.